The present invention relates to an arrangement for measuring the properties of multi-phase systems as well as the changes therein, such as changing interactions between particles in a solution. In particular, the invention relates to such an arrangement for use with multi-phase systems in which at least one of the phases is a liquid phase or fluid, such as gels, lattices, suspensions or emulsions.
The arrangement of the invention comprises a light source for producing a light beam. It is possible to use at least one source fibre having a first end arranged for receiving said laser beam and a second end arranged for emitting light in a multi-phase system, or to point the light directly into the multi-phase system. At least one detector fibre, arranged for detecting said light after being scattered by said multi-phase system, processing means arranged for receiving an output signal from said at least one detector fibre and for calculating predetermined parameters with respect to said multi-phase system.
Such an arrangement is known from D. S. Horne, Dynamic Light Scattering Studies of Concentrated Casein Micelle Suspensions, Chapter 15 in S. E. Hardings, e.a. Laser Light Scattering in Biochemistry, 1992.
The Diffusing Wave Spectroscopy (DWS) arrangement described by Horne, comprises a bifurcated optical fibre bundle as light guide. Half the fibres are connected to a randomly polarized Hexe2x80x94Ne laser. The other half of the fibres is connected to a photomultiplier. The bundle of fibres is distributed randomly over the face of a common leg. In use, the non-connected ends of the fibres are dipped into a scattering multi-phase system, e.g. milk or a milk derived medium/solution. Those fibres which are connected to the laser emit laser light into the multi-phase system. Light backscattered by the multi-phase system is detected by the fibres connected to the photomultiplier. Masking by a slit and a pinhole may ensure that light from only a small area impinges on the detector.
With such an arrangement, intensity correlation functions can be measured. Examples of such functions are presented for 330 nm polystyrene latex and undiluted skim milk. Moreover, Horne shows that relaxation time as a function of casein micelle volume fraction in reconstituted milk can be measured, Horne also shows that the relaxation time changes due to curd formation. Thus, the transition from fluid to gel can be detected. One of Horne""s conclusions is that: xe2x80x9cIt therefore appears that observation of DWS behaviour in these gelling systems, by virtue of its measurement of relaxation in the system, must eventually reflect changes in visco-elastic propertiesxe2x80x9d. However, Horne does not indicate how visco-elastic properties may be quantitatively derived from DWS measurements.
Moreover, the measurements described by Horne can not easily be made quantitative because these measurements are made with a randomly distributed bundle of incoming and outgoing optical fibres.
A. C. M. van Hooydonk, e.a., Control and Determination of the Curd-setting during Cheesemaking, Bulletin of the International Dairy Federation (1988) (No. 255), pp. 2-10, observe that a lot of scientific research has been devoted to rennet-induced coagulation of milk. However, up to now no quantitative measurement of gel formation and the subsequent synersis of the curd is available. The optimum coagulum firmness must be determined on-line for cutting to obtain maximum cheese yield and cheese quality. Hooydonk e.a. note: xe2x80x9cUp to now most cheesemakers judge the optimum cutting time by the xe2x80x9cfeelxe2x80x9d of the curd and . . . with amazing accuracyxe2x80x9d. Moreover, they note that although many instruments have been developed to carry out this task automatically, none of them have been widely accepted. The so-called xe2x80x9cGelographxe2x80x9d is considered as a standard instrument for measuring the gelation of cheesemilk at low gel strength. However, due to ongoing automation and increase of scale of cheesemaking plants there is a strong interest in automatic methods for monitoring the process of curd-setting.
In the U.S. Pat. No. 4,975,237 a dynamic light scattering apparatus is disclosed, comprising a laser as light source, optically coupled to a light scattering sample via a first optical fibre and a first lens. The lens produces a beam waist in a sample and scattered light is collected by a receive lens and a receive fibre. A photodetector detects light transmitted by the receive fibre and converts it in an electrical signal. The photodetector is connected to a correlator and computer. This correlater is not used for quantitative measurements of characteristics of the sample.
A primary object of the present invention is to provide an apparatus with which the properties of liquids, such as solutions, dispersions and emulsions can be measured and to relate physico-chemical properties to light scattering measurement in liquids.
For the purposes of the invention, the term xe2x80x9cliquidsxe2x80x9d comprises both heterogeneous systems which contain two distinct phases, such as a liquid phase and a suspended solid phase, two immiscible liquid phases, or an emulsified (liquid) phase in a liquid phase, as well as more homogeneous systems which are subject to phase changes, phase transitions or phase formation, such as systems in which gel formation, coagulation, aggregation or changes in viscosity can occur.
These homogenous or heterogenous systems can comprise organic, inorganic as well as biological media or components, aqueous systems or solutions, or systems of a mixed organic/inorganic and/or biological nature. In a particular embodiment, the multi-phase system is milk or a milk derived medium/solution, for instance as used in cheese-making.
A further object of the present invention is to provide a method which can be carried out by an apparatus according to the invention and which is able to provide physico-chemical properties of such liquids by means of diffusing wave spectroscopy. Such a method may be related to monitoring the renneting of cheesemilk during cheese-making, but is not restricted thereto.
Thus, the arrangement according to a first aspect of the present invention as defined above is characterized in that a processing means is arranged to calculate a maximum value of the mean square displacement  less than xcex94rm2 greater than  from the autocorrelation function g(2) as a function of time and the value of the physico-chemical property from said calculated maximum value of the mean square displacement  less than xcex94rm2 greater than .
In an embodiment the physico-chemical property is the gel-strength Gxe2x80x2 which is calculated using the following equation:       G    xe2x80x2    ≈                    k        B            ⁢      T              ξ      ⁢              ⟨                  Δ          ⁢                      xe2x80x83                    ⁢          r          ⁢                      2            m                          ⟩            
in which,
kbxc2x7T=thermal energy of particles in the gel;
"xgr"=size of a cluster in the gel.
The arrangement according to a second aspect of present invention is characterized in that a processing means is arranged to determine the half decay time as a function of time of the autocorrelation function and to determine the value of the physico-chemical property using a predetermined relation between the half decay time and the autocorrelation function. This value may be the gelstrength.
A further difference between the arrangement according to the invention and the Horne arrangement is the configuration of the optical fibres. Whereas the fibres in the Horne arrangement are distributed randomly and the mutual distances between the fibres is unknown, in the arrangement according to the invention the mutual distances between the detector fibres are predetermined. In order to facilitate the calculations the detector fibres are, preferably, single-mode fibres suitable for one specific monochromatic wavelength. The fibres are preferably set up in the so-called xe2x80x9cback-scattering geometryxe2x80x9d, which makes it possible to quantify the autocorrelation function that is measured. They may have the shape of a dipstick, so they can be easily stuck into any kind of liquid.
The advantage of DWS in respect to the gelograph is that changing interactions between the (droplets, bubbles or particles of the) phrases can be measured. In the case of renneting of cheesemilk the effects of the addition of rennet can be observed in a much earlier stage of the process. The method is non-destructive, because its working principle is not based on mechanical principles, but on the scattering of monochromatic light that does not damage the liquid.
Furthermore, the DWS can be used to do local measurements, which also makes it possible to probe inhomogeneities in samples. The volume that is probed by one pair of source fibre and detector fibres ranges from 1 nl to 1 l or more and can be located in an infinitely large volume. It depends on the distance between the source fibre and detector fibres.
In one embodiment of the arrangement according to the invention a source fibre and the at least one detector fibre are immersed in a milk with an addition of a rennet.
In order to automatically cut cheesemilk at the proper gel-strength, in a further embodiment the arrangement comprises a cutting machine coupled to the processing means, wherein the processing means are arranged to compare the gelstrength with a reference gelstrength, and to activate the cutting machine upon the gelstrength reaching the reference gelstrenth for cutting the gel.
Moreover, a third aspect of the invention is directed to a method for measuring the physico-chemical properties of liquids such as solutions, dispersions and emulsions, comprising the steps of:
producing a light beam (2)
emitting light in a liquid (7);
detecting said light after being scattered by said liquid (7);
converting the detected light in an electrical signal;
transmitting said electrical signal to processing means;
calculating the autocorrelation function g(2) in the time- or the frequency domain of said electrical signal characterized in that a maximum value of the mean square displacement  less than xcex94rm2 greater than  is calculated from the autocorrelation function g(2) (xcfx84), or from g(2) (xcexd) in a similar way, as a function of time and the value of said property is calculated from said calculated maximum value of the mean square displacement  less than xcex94rm2 greater than . The value may be the gelstrength which is calculated using the following equation:       G    xe2x80x2    ≈                    k        B            ⁢      T              ξ      ⁢              ⟨                  Δ          ⁢                      xe2x80x83                    ⁢          r          ⁢                      2            m                          ⟩            
in which,
kbxc2x7T=thermal energy of particles in the gel;
"xgr"=size of a cluster in the gel.
A fourth aspect of the invention is directed to a method for measuring physico-chemical properties of liquids such as solutions, dispersions and emulsions, comprising the steps of:
producing a light beam (2)
emitting light in a liquid (7);
detecting said light after being scattered by said liquid (7);
converting the detected light in an electrical signal;
transmitting said electrical signal to processing means;
calculating the autocorrelation function g(2) of said electrical signal, characterized in that the half decay time is calculated as a function of time of the autocorrelation function and the value of said property is determined using a predetermined relation between the half decay time and the autocorrelation function. The value may be the gelstrength.